Infrared intruder detection system

Abstract

An infrared intruder detection system has two separate channels A and B. Each channel has its own pyroelectric detector device and threshold level comparator. The comparator is responsive to dual polarity pyroelectric voltage outputs produced by the associated detector device as a result of an intruder-related infrared image moving thereacross. A logic circuit analyzes the comparator outputs and responds to the identification of particular intruder-indicative sequences of comparator outputs in both channels to generate an alarm and/or switching output. The logic circuit may also identify shock-induced comparator outputs. The system offers a high intruder detection capability with immunity from false triggering.

We claim: 
     
       1. An infrared intruder detection system comprising: a first means for sensing infrared radiation incident on a first location, said first infrared-sensing means producing either a positive or negative output signal +A or -A in response to the infrared radiation sensed;   a second means for sensing infrared radiation incident on a second location spaced from the first location, said second infrared-sensing means producing either a positive or a negative output signal +B or -B in response to the infrared radiation sensed; and   sequence detection means for receiving the output signals from the first and second means for sensing infrared radiation, and for producing an alarm output indicating the presence of an intruder when an output signal sequence of +A followed by +A and +B, followed by +B is detected, or when an output signal sequence of -A, followed by -A and -B, followed by -B is detected.   
     
     
       2. An infrared intruder detection system as claimed in claim 1, characterized in that the sequence detection means also produces an alarm output indicating the presence of an intruder when an output signal sequence of +B followed by +A and +B, followed by +A is detected, or when an output signal sequence of -B, followed by -A and -B, followed by -A is detected. 
     
     
       3. An infrared intruder detection system as claimed in claim 2, characterized in that the sequence detection means also produces an alarm output indicating the presence of an intruder when an output signal sequence of +A followed by +A and +B, followed by +A is detected, or when an output signal sequence of -A, followed by -A and -B, followed by -A is detected. 
     
     
       4. An infrared intruder detection system as claimed in claim 3, characterized in that the sequence detection means also produces an alarm output indicating the presence of an intruder when an output signal sequence of +B, followed by +A and +B, followed by +B is detected, or when an output signal sequence of -B, followed by -A and -B, followed by -B is detected. 
     
     
       5. An infrared intruder detection system as claimed in claim 4, characterized in that the sequence detection means will not produce an alarm output indicating the presence of an intruder when an output signal sequence of -A and +B is detected, or when an output signal sequence of +A and -B is detected. 
     
     
       6. An infrared intruder detection system as claimed in claim 5, characterized in that the system further comprises means for defining a timing period window within which the sequence detection means must detect a selected output signal sequence in order to produce an alarm output. 
     
     
       7. An infrared intruder detection system as claimed in claim 6, characterized in that the first and second locations are interdigitated. 
     
     
       8. An infrared intruder detection system as claimed in claim 7, characterized in that each means for sensing infrared radiation comprises a pyroelectric dual detector having differentially-connected pyroelectric elements. 
     
     
       9. An infrared intruder detection system as claimed in claim 8, characterized in that the four elements of the two dual detectors are arranged in a line. 
     
     
       10. An infrared intruder detection system as claimed in claim 9, characterized in that the system further comprises means for generating an alarm in response to the alarm output of the sequence detection means. 
     
     
       11. An infrared intruder detection system as claimed in claim 9, characterized in that the system further comprises means for operating a switch in response to the alarm output of the sequence detection means.